Recently as optical materials, plastics have been predominantly used because they are light-weight and resistant to fracture in comparison with glasses and readily stainable. For example, polyethyleneglycolbisallyl carbonates, polymethyl methacrylates, alicyclic polyolefins and polycarbonates have been commonly used as an optical resin. However, these have a refractive index of 1.6 or less, so that when they are used for coating or bonding of a material having a high refractive index, the intrinsic properties of the object adhered cannot be fully utilized and furthermore, when they are used for, for examples a lens, a lens thickness becomes larger. Since a plastic having a higher refractive index generally tends to have a lower Abbe's number and a larger chromatic aberration, an observed object looks tinted and blurred. Thus, a plastic having a high refractive index and a high Abbe's number has been needed as an optical resin. When being used for, for example, a lens, a plastic must be furthermore resistant to deformation within use temperature range, and a plastic for an optical use is, therefore, additionally needed to have a high glass transition temperature.
As such an optical resin, there have been proposed ring-opening polymers of a polyepisulfide compound in, for example, Patent documents 1 and 2. However, such a polymer having a higher glass transition temperature tends to have a smaller bending distortion, leading to its fragility, and, therefore, there has been a resin having a high refractive index, a high Abbe's number and furthermore a high glass transition temperature which is endowed with flexibility to a bending stress.
Furthermore, Patent document 3 has proposed a polythioether or sulfur-containing polyether as a ring-opening polymer of a poly(thio)epoxy compound having a trithiocarbonate moiety. This polymer is, however, unsatisfactory because it has insufficient flexibility to a bending stress and the poly(thio)epoxy compound has in adequate hue (APHA of considerably larger than 200) in the light of transparency.
Furthermore, Patent documents 4 to 6 and Non-Patent documents 1 and 2 have proposed, as an optical resin, a polythiocarbonate prepared by reacting an aliphatic (including alicyclic) dithiol and phosgene. However, this polymer has a chlorine atom at a molecular terminal and has a higher melting point, which become drawbacks when the polymer is to be further used a starting material for an optical resin by melt molding or cast polymerization. In terms of a practical manufacturing process, there is a problem that a highly toxic phosgene is used.
Patent document 1: Japanese Laid-open Patent Publication No. 1997-71580;
Patent document 2: Japanese Laid-open Patent Publication No. 1997-110979;
Patent document 3: Japanese Laid-open Patent Publication No. 2000-53761;
Patent document 4: Japanese Laid-open Patent Publication No. 2002-201277;
Patent document 5: Japanese Laid-open Patent Publication No. 2005-29608;
Patent document 6: Japanese Laid-open Patent Publication No. 2005-31175;
Non-Patent document 1: POLYMER, 35, 7, 1564 (1994);
Non-Patent document 2: POLYMER COMMUNICATIONS, 1990, 31, 431